theme/powerline: fix an oops in the last patch

The tilde should not have been escaped, and in fact I did not have it escaped in my main branch, but the PR I submitted did have it escaped and...now it shows up in the prompt line for all the PowerLine themes... oops.

themes/powerline/powerline.base.bash

@@ -142,7 +142,7 @@ function __powerline_scm_prompt() {
 }
 
 function __powerline_cwd_prompt() {
-	local cwd="${PWD/$HOME/\~}"
+	local cwd="${PWD/$HOME/~}"
 
 	echo "${cwd}|${CWD_THEME_PROMPT_COLOR}"
 }