fix: Match solution files with exercise files

[JatinSanghvi]

This PR updates comments within solutions directory to match with the ones in exercise.

• No functional changes

Changes Made

Matching exercise files

• hashmaps1.rs: Replaced "e.g apple" with "e.g. apple"
• errors5.rs: Fixed capitalization - "To do so, The" -> "To do so, the"
• options1.rs: Renamed maybe_icecream to maybe_ice_cream

Other changes

• strings4.rs: Fixed grammar - "kept &str" -> "kept as &str"
• macros3.rs: Removed redundant word - "attribute macro_use attribute" -> "macro_use attribute"
• lifetimes2.rs: Added spaces - "Solution1/2" -> "Solution 1/2". This follows the convention used in another file.

[mo8it]

Thanks :)