docs: Update substr usage in Velox developer doc vector section (facebookincubator#12979)

Summary:
Pull Request resolved: facebookincubator#12979

As onboarding reading the velox developer document, basing on the context, adding presto `substr(s,2)`
 {F1976936850}

Reviewed By: kKPulla

Differential Revision: D72726144

fbshipit-source-id: 2074ba0468be25b048f9a03e0b0ced66128c4441

Author: juwentus1234

velox/docs/develop/vectors.rst

@@ -242,8 +242,8 @@ like substr and split. The results of these functions consist of substrings of
 the original strings and therefore can use StringViews which point to the same
 string buffers as the input vectors.
 
-A result of applying substr(s, 2) function to a vector shown above looks like
-this:
+A result of applying `presto substr(s, 2) <https://prestodb.io/docs/current/functions/string.html#substr-string-start-varchar>`_ function to a vector shown above looks
+like this:
 
 .. image:: images/substr-result.png
     :width: 700
@@ -252,8 +252,8 @@ this:
 This vector is using the same string buffer as the original one. It simply
 references it using std::shared_ptr. The individual StringView entries either
 contain strings inline or refer to positions in the original strings buffer.
-After applying substr(s, 2) function a string in position 1 became short enough
-to fit inside the StringView, hence, it no longer contains a pointer to a
+After applying `presto substr(s, 2) <https://prestodb.io/docs/current/functions/string.html#substr-string-start-varchar>`_ function a string in position 1 became short
+enough to fit inside the StringView, hence, it no longer contains a pointer to a
 position in the string buffer.
 
 Allowing these zero-copy implementations of functions that simply change the