necessities: Use "MIT OR Apache-2.0"

Catch up with rust-lang/cargo@7dee65fe (src/doc/manifest: Pin
'license' to SPDX 2.1 expressions and the 2.4 list, 2018-01-04,
rust-lang/cargo#4898), which deprecated '/' in favor of vanilla SPDX
license expressions.

src/necessities.md

@@ -66,7 +66,7 @@ To apply the Rust license to your project, define the `license` field in your
 name = "..."
 version = "..."
 authors = ["..."]
-license = "MIT/Apache-2.0"
+license = "MIT OR Apache-2.0"
 ```
 
 And toward the end of your README.md: