struct module minor documentation fix

[vsky279]

Fixes - no reported issue.

• This PR is for the dev branch rather than for master.
• This PR is compliant with the other contributing guidelines as well (if not, please describe why).
• I have thoroughly tested my contribution.
• The code changes are reflected in the documentation at docs/*.

Precises documentation of the struct module.

 ## struct.unpack()
 
 Returns the values packed in string `s` according to the format
-string `fmt`. An optional `i` marks where in `s` to start reading
+string `fmt`. An optional `offset` marks where in `s` to start reading
 (default is 1). After the read values, this function also returns
 the index in `s` where it stopped reading, which is also where you
 should start to read the rest of the string.

 #### Syntax

 `struct.unpack (fmt, s[, offset])`
 
 #### Returns
 
-All the unpacked data.
+All the unpacked data and index in `s` where it stoppped reading.