Can't find rows starting by space using `^ ` regular expression

[Norbert]

#### Expected behaviour
Pluma finds rows which start with a space.

#### Actual behaviour
Pluma can't find rows which start with a space.

#### Steps to reproduce the behaviour
1. Create simple test document by using command below `echo -e "\t1tab\t1tab\n 2space 2space" > /tmp/file`
2. Open this file in Pluma by `pluma /tmp/file`
3. Press <Ctrl>+<F>, check *Match regular expression* and *Parse escape sequences (e.g. \n)*, put `^ ` in *search for*
4. Click *find*

#### MATE general version
1.24.1

#### Package version
1.24.2-0ubuntu1

#### Linux Distribution
Ubuntu MATE 21.04

ProblemType: Bug
DistroRelease: Ubuntu 21.04
Package: pluma 1.24.2-0ubuntu1
ProcVersionSignature: Ubuntu 5.11.0-18.19-generic 5.11.17
Uname: Linux 5.11.0-18-generic x86_64
ApportVersion: 2.20.11-0ubuntu65.1
Architecture: amd64
CasperMD5CheckResult: pass
CurrentDesktop: MATE
Date: Sun Jul 4 00:02:43 2021
InstallationDate: Installed on 2021-04-23 (71 days ago)
InstallationMedia: Ubuntu-MATE 21.04 "Hirsute Hippo" - Release amd64 (20210420)
SourcePackage: pluma
UpgradeStatus: No upgrade log present (probably fresh install)