Grammar fix in lifetime syntax (#208)

I'm guessing there used to be two different reference examples here and one was removed, but now there's a subject-verb number mismatch. (Maybe the sentence should be rewritten entirely though.)

src/ch10-03-lifetime-syntax.md

@@ -461,7 +461,7 @@ which it can’t figure out lifetimes, the compiler will stop with an error.
 These rules apply to `fn` definitions as well as `impl` blocks.
 
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-The first rule is that the compiler assigns a different lifetime parameter to each lifetime in each input type. References like `&'_ i32` needs a lifetime parameter, and structures like `ImportantExcerpt<'_>` need a lifetime parameter. For example:
+The first rule is that the compiler assigns a different lifetime parameter to each lifetime in each input type. References like `&'_ i32` need a lifetime parameter, and structures like `ImportantExcerpt<'_>` need a lifetime parameter. For example:
 * The function `fn foo(x: &i32)` would get one lifetime parameter and become `fn foo<'a>(x: &'a i32)`. 
 * The function `fn foo(x: &i32, y: &i32)` would get two lifetime parameters and become `fn foo<'a, 'b>(x: &'a i32, y: &'b i32)`.
 * The function `fn foo(x: &ImportantExcerpt)` would get two lifetime parameters and become `fn foo<'a, 'b>(x: &'a ImportantExcerpt<'b>)`.