Documentation: git-clean: fix minor grammatical errors

There were a few minor grammatical errors that made this paragraph hard
to read. This patch fixes the errors in a very minimal manner.

Signed-off-by: Wesley J. Landaker <wjl@icecavern.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-clean.txt

@@ -12,9 +12,9 @@ SYNOPSIS
 
 DESCRIPTION
 -----------
-Removes files unknown to git.  This allows to clean the working tree
-from files that are not under version control.  If the '-x' option is
-specified, ignored files are also removed, allowing to remove all
+Removes files unknown to git.  This allows cleaning the working tree
+of files that are not under version control.  If the '-x' option is
+specified, ignored files are also removed, allowing the removal of all
 build products.
 If any optional `<path>...` arguments are given, only those paths
 are affected.