Improve auto-generated title in JSON schemas

[skewty]

Underscore is converted to space.
Closes #743

NOTE the following unit test is not passing for me:

tests/test_validators.py:641 (test_assert_raises_validation_error)
def test_assert_raises_validation_error():
        class Model(BaseModel):
            a: str
    
            @validator('a')
            def check_a(cls, v):
                assert v == 'a', 'invalid a'
                return v
    
        Model(a='a')
    
        with pytest.raises(ValidationError) as exc_info:
            Model(a='snap')
        injected_by_pytest = "\nassert 'snap' == 'a'\n  - snap\n  + a"
>       assert exc_info.value.errors() == [
            {'loc': ('a',), 'msg': f'invalid a{injected_by_pytest}', 'type': 'assertion_error'}
        ]
E       assert [{'loc': ('a',),\n  'msg': "invalid a\nassert 'snap' == 'a'",\n  'type': 'assertion_error'}] == [{'loc': ('a',),\n  'msg': "invalid a\nassert 'snap' == 'a'\n  - snap\n  + a",\n  'type': 'assertion_error'}]

tests/test_validators.py:656: AssertionError

[samuelcolvin]

Looks good to me, please add a change description.

In general best to keep the checkboxes from the PR template, but doesn't matter this time.