Fixed some grammatical errors in git-rebase.txt documentation.

Generally, the dependent clause "for example" is suffixed with a comma.
Used present tense where appropriate to be consistent with the other
paragraphs.

Rewrote the paragraph in the second hunk to be more clear.

Signed-off-by: Garry Dolley <gdolley@ucla.edu>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

Documentation/git-rebase.txt

@@ -92,7 +92,7 @@ branch to another, to pretend that you forked the topic branch
 from the latter branch, using `rebase --onto`.
 
 First let's assume your 'topic' is based on branch 'next'.
-For example feature developed in 'topic' depends on some
+For example, a feature developed in 'topic' depends on some
 functionality which is found in 'next'.
 
 ------------
@@ -103,9 +103,9 @@ functionality which is found in 'next'.
                             o---o---o  topic
 ------------
 
-We would want to make 'topic' forked from branch 'master',
-for example because the functionality 'topic' branch depend on
-got merged into more stable 'master' branch, like this:
+We want to make 'topic' forked from branch 'master'; for example,
+because the functionality on which 'topic' depends was merged into the
+more stable 'master' branch. We want our tree to look like this:
 
 ------------
     o---o---o---o---o  master