docs(limitToFilter): mark the begin param as optional

Closes angular#11094

Author: gkalpak

src/ng/filter/limitTo.js

@@ -17,7 +17,7 @@
  *     If the number is negative, `limit` number  of items from the end of the source array/string
  *     are copied. The `limit` will be trimmed if it exceeds `array.length`. If `limit` is undefined,
  *     the input will be returned unchanged.
- * @param {string|number} begin Index at which to begin limitation. As a negative index, `begin`
+ * @param {(string|number)=} begin Index at which to begin limitation. As a negative index, `begin`
  *     indicates an offset from the end of `input`. Defaults to `0`.
  * @returns {Array|string} A new sub-array or substring of length `limit` or less if input array
  *     had less than `limit` elements.