Fix syntax error in colored-man-pages

Commit 09d2a597 introduced the bug, which is due to using
single brackets conditions at the same time as *.

This commit should fix it and use double brackets everywhere
else.

Fixes #4699

plugins/colored-man-pages/colored-man-pages.plugin.zsh

-if [ "$OSTYPE" = solaris* ]
+if [[ "$OSTYPE" = solaris* ]]
 then
-	if [ ! -x "$HOME/bin/nroff" ]
+	if [[ ! -x "$HOME/bin/nroff" ]]
 	then
 		mkdir -p "$HOME/bin"
 		cat > "$HOME/bin/nroff" <<EOF