Fix markdown lint issues in nginx.md

[kadler]

nginx.md:3 MD022/blanks-around-headings/blanks-around-headers Headings should be surrounded by blank lines [Expected: 1; Actual: 0; Below] [Context: "## Summary"]
nginx.md:4:81 MD013/line-length Line length [Expected: 80; Actual: 168]
nginx.md:8 MD040/fenced-code-language Fenced code blocks should have a language specified [Context: "```"]
nginx.md:9:1 MD014/commands-show-output Dollar signs used before commands without showing output [Context: "$ yum install nginx"]
nginx.md:12 MD022/blanks-around-headings/blanks-around-headers Headings should be surrounded by blank lines [Expected: 1; Actual: 0; Below] [Context: "## Config"]
nginx.md:14 MD031/blanks-around-fences Fenced code blocks should be surrounded by blank lines [Context: "```"]
nginx.md:14 MD040/fenced-code-language Fenced code blocks should have a language specified [Context: "```"]
nginx.md:29 MD022/blanks-around-headings/blanks-around-headers Headings should be surrounded by blank lines [Expected: 1; Actual: 0; Below] [Context: "## Starting and Stopping"]
nginx.md:30:81 MD013/line-length Line length [Expected: 80; Actual: 206]
nginx.md:32 MD040/fenced-code-language Fenced code blocks should have a language specified [Context: "```"]
nginx.md:33:1 MD014/commands-show-output Dollar signs used before commands without showing output [Context: "$ nginx -c /www/nginx/nginx.co..."]
nginx.md:38 MD040/fenced-code-language Fenced code blocks should have a language specified [Context: "```"]
nginx.md:39:1 MD014/commands-show-output Dollar signs used before commands without showing output [Context: "$ nginx -c /www/nginx/nginx.co..."]
nginx.md:44:81 MD013/line-length Line length [Expected: 80; Actual: 217]
nginx.md:46 MD012/no-multiple-blanks Multiple consecutive blank lines [Expected: 1; Actual: 2]