Update RFC 496, typos, use correct field name (strict => strictMode)

[NullVoxPopuli]

all the implementations of strict-mode stuff use strictMode, rather than just strict
(that I could find)
ember-template-imports, here:

• https://github.com/ember-template-imports/ember-template-imports/blob/21c232c629e9d48b18b42a69d0617fd8c62c2549/src/preprocessor-plugin.js#L41
• https://github.com/ember-template-imports/ember-template-imports/blob/21c232c629e9d48b18b42a69d0617fd8c62c2549/src/preprocess-embedded-templates.ts#L133

ember-source:

• https://github.com/emberjs/ember.js/blob/master/types/preview/%40ember/template-compilation/index.d.ts#L18
• https://github.com/emberjs/ember.js/blob/7c6fec2405516a98a6f59fc31b46056dbeb59d09/packages/ember-template-compiler/lib/system/compile-options.ts#L52
• https://github.com/emberjs/ember.js/blob/7c6fec2405516a98a6f59fc31b46056dbeb59d09/packages/internal-test-helpers/lib/compile.ts#L33

The <template> RFC: https://github.com/emberjs/rfcs/pull/779/files#diff-9a7c2cf97ea750d29eb74c905ba69631d23b9f30f868d93ec9746f30840cfaa6R330

@glimmer/compiler: https://github.com/glimmerjs/glimmer-vm/blob/master/packages/%40glimmer/compiler/lib/compiler.ts#L124

I thought it'd be easier to update this RFC than to update all the implementations to use strict instead of strictMode -- also felt like it'd be the stabler option, as gjs/gts is presently emitted with strictMode

[ef4]

I think the RFC deliberately picked strict and the experimental implementations that happened before the RFC were expected to change.