More typos found by misspell

Install https://github.com/client9/misspell, run
```
git ls-files '*.scala'|egrep -v 'scala2-library|scala-backend|collection-strawman'|xargs misspell
```
and review output.

Author: Blaisorblade

doc-tool/src/dotty/tools/dottydoc/util/MemberLookup.scala

@@ -16,7 +16,7 @@ import model._
 trait MemberLookup {
   /** Performs a lookup based on the provided (pruned) query string
    *
-   *  Will return a `Tooltip` if unsucessful, otherwise a LinkToEntity or
+   *  Will return a `Tooltip` if unsuccessful, otherwise a LinkToEntity or
    *  LinkToExternal
    */
   def lookup(entity: Entity, packages: Map[String, Package], query: String): Option[Entity] = {

tests/pos/sams.scala

@@ -69,7 +69,7 @@ object test {
 // synthesizeSAMFunction where the sam type is not fully defined
 class T {
   trait F[T, U] { def apply(x: T): U }
-  // this is an inner trait, that will recieve an abstract $outer pointer. Not a SAM.
+  // this is an inner trait, that will receive an abstract $outer pointer. Not a SAM.
   def app[T, U](x: T)(f: F[T, U]): U = f(x)
   app(1)(x => List(x))
 }