tools/trim: fix package doc

[twpayne]

Currently the doc for tools/trim at https://godoc.org/cuelang.org/go shows:

light: ceiling50: { room: "MasterBedroom" brightnessOff: 0.0 // this line brightnessOn: 100.0 // and this line will be removed }

This is due to a couple of blank lines in the tools/trim package comment.

This PR fixes that, and a couple of typos.

[myitcv]

LGTM. FYI - the diff pre post of go doc is as follows:

--- pre 2020-05-16 07:53:38.334860677 +0100
+++ post        2020-05-16 07:53:33.803031316 +0100
@@ -1,11 +1,40 @@
 package trim // import "cuelang.org/go/tools/trim"

+Package trim removes definitions that may be inferred from templates.
+
+A field, struct, or list is removed if it is implied by a constraint, such
+as from an optional field matching a required field, a list type value, a
+comprehension or any other implied content. It will modify the files in
+place.
+
+
+Limitations
+
+Removal is on a best effort basis. Some caveats: - Fields in implied content
+may refer to fields within the struct in which
+
+    they are included, but are only resolved on a best-effort basis.
+
+- Disjunctions that contain structs in implied content cannot be used to
+
+    remove fields.
+
+- There is currently no verification step: manual verification is required.
+
+Examples:
+
+    light: [string]: {
+       room:          string
+       brightnessOff: *0.0 | >=0 & <=100.0
+       brightnessOn:  *100.0 | >=0 & <=100.0
+    }

     light: ceiling50: {
        room:          "MasterBedroom"
        brightnessOff: 0.0    // this line
        brightnessOn:  100.0  // and this line will be removed
     }
+
 Results in:

     light: [string]: {

[myitcv]

cc @mpvl