Improve TS instructions (#3414)

The regex for TS transformer in the doc is too broad. For example, I set
the regex to `".(ts|tsx)"` as recommended in the docs and my test would
fail. The reason is that  one of my tested file imports the file `isArguments.js`
from `lodash-es` (ES 2015 build of lodash). Because `isArguments.js` matches the pattern
`.(ts|tsx)`, this file is not transpiled by babel (even after I've set
`transformIgnorePatterns` to `"<rootDir>/node_modules/(?!lodash-es)"`.
Changing the pattern to `"^.+\\.tsx?$"` fixes this problem.

README.md

@@ -116,7 +116,7 @@ then modify your `package.json` so the `jest` section looks something like:
 {
   "jest": {
     "transform": {
-      ".(ts|tsx)": "<rootDir>/node_modules/ts-jest/preprocessor.js"
+      "^.+\\.tsx?$": "<rootDir>/node_modules/ts-jest/preprocessor.js"
     },
     "testRegex": "(/__tests__/.*|\\.(test|spec))\\.(ts|tsx|js)$",
     "moduleFileExtensions": [

docs/en/GettingStarted.md

@@ -120,7 +120,7 @@ then modify your `package.json` so the `jest` section looks something like:
 {
   "jest": {
     "transform": {
-      ".(ts|tsx)": "<rootDir>/node_modules/ts-jest/preprocessor.js"
+      "^.+\\.tsx?$": "<rootDir>/node_modules/ts-jest/preprocessor.js"
     },
     "testRegex": "(/__tests__/.*|\\.(test|spec))\\.(ts|tsx|js)$",
     "moduleFileExtensions": [