docs(numberFilter): improve wording for infinity description

Closes angular#14100

Author: nonopolarity

src/ng/filter/filters.js

@@ -85,7 +85,7 @@ function currencyFilter($locale) {
  * Formats a number as text.
  *
  * If the input is null or undefined, it will just be returned.
- * If the input is infinite (Infinity/-Infinity) the Infinity symbol '∞' is returned.
+ * If the input is infinite (Infinity or -Infinity), the Infinity symbol '∞' or '-∞' is returned, respectively.
  * If the input is not a number an empty string is returned.
  *
  *