doc: Trim .txt files trailing whitespace

via OSX: find . -name '*.txt' -exec sed -i '' -E 's/[[:space:]]+$//' {} +

doc/nimgrep.txt

@@ -7,11 +7,11 @@
 
 
 Nimgrep is a command line tool for search&replace tasks. It can search for
-regex or peg patterns and can search whole directories at once. User 
+regex or peg patterns and can search whole directories at once. User
 confirmation for every single replace operation can be requested.
 
-Nimgrep has particularly good support for Nim's 
-eccentric *style insensitivity*. Apart from that it is a generic text 
+Nimgrep has particularly good support for Nim's
+eccentric *style insensitivity*. Apart from that it is a generic text
 manipulation tool.
 
 
@@ -34,10 +34,10 @@ Options:
   --find, -f          find the pattern (default)
   --replace, -r       replace the pattern
   --peg               pattern is a peg
-  --re                pattern is a regular expression (default); extended 
+  --re                pattern is a regular expression (default); extended
                       syntax for the regular expression is always turned on
   --recursive         process directories recursively
-  --confirm           confirm each occurrence/replacement; there is a chance 
+  --confirm           confirm each occurrence/replacement; there is a chance
                       to abort any time without touching the file
   --stdin             read pattern from stdin (to avoid the shell's confusing
                       quoting rules)