docs : add a comma in structs3.rs file

[Allupeng]

trafficstars

when i use rustRover, and there is a grammar error says "Use a comma before 'and' if it connects two independent clauses (unless they are closely connected and short). ". So add a comma in structs3.rs file will solve this error.

[Allupeng]

and i find another grammar in hashmaps1.rs file which is e.g , the correct should be e.g.

[mo8it]

I decided to remove the comma because I am not sure about it. But thank for adding the dot :D

[Allupeng]

I decided to remove the comma because I am not sure about it. But thank for adding the dot :D

hopefully i could help rust community :)