chore: capitalize c letter

By capitalizing the `c` letter it makes clear that we're talking about the C programming language.

Author: chapeupreto

exercises/structs/README.md

@@ -1,6 +1,6 @@
 ### Structs
 
-Rust has three struct types: a classic c struct, a tuple struct, and a unit struct.
+Rust has three struct types: a classic C struct, a tuple struct, and a unit struct.
 
 #### Book Sections