Missed some keep/discard DF examples in #48

[ruebot]

Looks like we still have some keep/discard examples with DataFrames that are still in the docs, and should have been taken care of in #48

current/text-analysis.md
32:  .keepValidPages()
68:  .keepValidPages()
106:  .keepValidPages()
107:  .keepDomains(Set("www.archive.org"))
120:  .keepDomainsDF(Set("www.archive.org"))
145:  .keepValidPages()
146:  .keepUrlPatterns(Set("(?i)http://www.archive.org/details/.*".r))
159:  .keepUrlPatternsDF(Set("(?i)http://www.archive.org/details/.*".r))
183:  .keepValidPages()
184:  .keepDomains(Set("www.archive.org"))
197:  .keepDomainsDF(Set("www.archive.org"))
211:conceives of the date string as a `DateComponent`. Use `keepDate` to specify
223:  .keepValidPages()
224:  .keepDate(List("200804"), ExtractDateRDD.DateComponent.YYYYMM)
237:  .keepValidPages()
238:  .keepDate(List("2008"), ExtractDateRDD.DateComponent.YYYY)
251:  .keepValidPages()
252:  .keepDate(List("2008","2015"), ExtractDateRDD.DateComponent.YYYY)
276:  .keepDateDF(List("2008","2015"), "YYYY")
289:The following Spark script keeps only French language pages from a certain
298:  .keepValidPages()
299:  .keepDomains(Set("www.archive.org"))
300:  .keepLanguages(Set("fr"))
313:  .keepDomainsDF(Set("www.archive.org"))
314:  .keepLanguagesDF(Set("fr"))
325:  .keepDomainsDF(Set("www.archive.org"))
339:The following Spark script keeps only pages containing a certain keyword, which
350:  .keepValidPages()
351:  .keepContent(Set("radio".r))
368:  .keepContentDF(Set("radio".r))
393:  .keepValidPages()

current/collection-analysis.md
24:RecordLoader.loadArchives("/path/to/warcs", sc).keepValidPages()
67:RecordLoader.loadArchives("/path/to/warcs", sc).keepValidPages()
114:RecordLoader.loadArchives("/path/to/warcs", sc).keepValidPages()
139:  .keepUrlPatternsDF(urlPattern)
192:RecordLoader.loadArchives("/path/to/warcs", sc).keepValidPages()
205:RecordLoader.loadArchives("/path/to/warcs", sc).keepValidPages()

current/link-analysis.md
38:  .keepValidPages()
49:can be applied immediately after `.keepValidPages()`.
57:  .keepValidPages()
58:  .keepContent(Set("apple".r))
87:  .keepContentDF(Set("apple".r))
112:  .keepValidPages()
158:using the `keepUrlPatterns` command.
166:  .keepValidPages()
167:  .keepUrlPatterns(Set("(?i)http://www.archive.org/details/.*".r))
184:  .keepUrlPatternsDF(Set("(?i)http://www.archive.org/details/.*".r))
213:RecordLoader.loadArchives("/path/to/warcs", sc).keepValidPages()
278:URL pattern listed under `keepUrlPatterns`.
285:  .keepValidPages()
286:  .keepUrlPatterns(Set("http://www.archive.org/details/.*".r))
303:  .keepUrlPatternsDF(Set("http://www.archive.org/details/.*".r))
330:  .keepValidPages()

current/toolkit-walkthrough.md
109:  .keepValidPagesDF()
121:- tells it only to keep the
122:  "[valid](https://github.com/archivesunleashed/aut-docs/blob/master/aut-0.50.0/filters.md#keep-valid-pages)"
178:  .keepValidPagesDF()
205:  .keepDomainsDF(domains)
237:  .keepDomainsDF(domains)
260:in and around the `.keepDomainsDF` line. Check out the
266:- **Keep URL Patterns**: Instead of domains, what if you wanted to have text
267:  relating to just a certain pattern? Substitute `.keepDomainsDF` for a command
269:  `.keepUrlPatternsDF(Set("(?i)http://geocities.com/EnchantedForest/.*".r))`
271:  following command after `.webpages()`: `.keepDateDF(List("2006"), "YYYY")`
273:  `.keepDomainsDF` add a new line: `.keepLanguagesDF(Set("fr"))`.
286:  .keepDomainsDF(domains)
287:  .keepLanguagesDF(languages)
300:  .keepDateDF(List("2006"), "YYYY")
369:  .keepValidPages()
379:By now this should be seeming pretty straightforward! (remember to keep using