fix: correctly tab-complete nicks starting with `$`

Without it, nicks starting with `$` would have doubled `$`.

Continuation of #1377

Pointed out by @ProMcTagonist in #1167

src/widget/form/tabcompleter.cpp

@@ -54,7 +54,8 @@ void TabCompleter::buildCompletionList()
     nextCompletion = completionMap.begin();
 
     // split the string on the given RE (not chars, nums or braces/brackets) and take the last section
-    QString tabAbbrev = msgEdit->toPlainText().left(msgEdit->textCursor().position()).section(QRegExp("[^\\w\\d:@--_\\[\\]{}|`^.\\\\]"), -1, -1);
+    QString tabAbbrev = msgEdit->toPlainText().left(msgEdit->textCursor().position())
+        .section(QRegExp("[^\\w\\d\\$:@--_\\[\\]{}|`^.\\\\]"), -1, -1);
     // that section is then used as the completion regex
     QRegExp regex(QString("^[-_\\[\\]{}|`^.\\\\]*").append(QRegExp::escape(tabAbbrev)), Qt::CaseInsensitive);