Docs: correct small grammatical error in read.rst

[asfaltboy]

Description

Just noticed this sentence doesn't make sense (the construct is typically used as 'while something, something else' but we just end the sentence early); I assumed it intended to reference the while command, so I attempted to link to it to help the reader to read more if they wished so.

Fixes issue #

TODOs:

• Changes to fish usage are reflected in user documentation/manpages.
• Tests have been added for regressions fixed
• User-visible changes noted in CHANGELOG.rst

[faho]

The intended construct is "The following code stores the value 'hello' in $foo: (some code), while this code here can be used to read line-by-line: (some more code)".

Which is a tad awkward so let's use this PR instead. Thanks, merged!