docs: change reference from RxJS "lettable" to "pipeable"

Author: CaerusKaru

CHANGELOG.md

@@ -6,7 +6,7 @@ The 2.0.0-beta.10 and beta.11 releases on npm accidentally glitched-out midway,
 ### Build Changes
 
 Similar to those used in **@angular/material**, this release also upgrades the package dependencies to Angular ~5.1.0 and the RxJS 5.5.x. 
->  [Lettable RxJS operators](https://github.com/ReactiveX/rxjs/blob/master/doc/lettable-operators.md) are now used instead of prototype-patching *add* operators. 
+>  [Pipeable RxJS operators](https://github.com/ReactiveX/rxjs/blob/master/doc/pipeable-operators.md) are now used instead of prototype-patching *add* operators. 
 
 * **packages:** update deps to Angular v5 and RxJs 5.5 ([#523](https://github.com/angular/flex-layout/pull/523)) ([62457a5972](https://github.com/angular/flex-layout/commit/62457a5972)), closes [#519](https://github.com/angular/flex-layout/issues/519)
 

CODING_STANDARDS.md

@@ -127,7 +127,7 @@ class ConfigBuilder {
 ```
 
 #### RxJS
-When dealing with RxJS operators, import the lettable operator (e.g.
+When dealing with RxJS operators, import the pipeable operator (e.g.
 `import {map} from 'rxjs/operators/map'`), as opposed to using the "patch" imports which pollute the
 user's global Observable object (e.g. `import 'rxjs/add/operator/map'`):