ports.7: Fix example of passing variables on the command line

make(1)'s -D flag does not allow for setting the value of the variable.
It just defines the variable and sets its value to 1. In fact, make(1)
treats "=" as just another character in the variable name:

```
$ make -DA=2 -V A  # Output is just an empty line.

$ make -DA=2 -V A=2  # Variable "A=2" is defined and set to "1".
1
```

Fixes:	d25f7d324a9d ports.7: Document DEBUG_FLAGS and the process of debugging ports
MFC after:	3 days

1 files changed, 2 insertions, 2 deletions

diff --git a/share/man/man7/ports.7 b/share/man/man7/ports.7
index b681a326f407..75070ce852fe 100644
--- a/share/man/man7/ports.7
+++ b/share/man/man7/ports.7
@@ -25,7 +25,7 @@
 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
-.Dd March 21, 2025
+.Dd September 10, 2025
 .Dt PORTS 7
 .Os
 .Sh NAME
@@ -683,7 +683,7 @@ WITH_DEBUG_PORTS=	mail/dovecot security/krb5
 .Pp
 It is also possible to use the debug variables on the command line:
 .Bd -literal -offset 2n
-.Li # Ic make -DWITH_DEBUG DEBUG_FLAGS="-g -O0" build
+.Li # Ic make WITH_DEBUG DEBUG_FLAGS="-g -O0" build
 .Ed
 .Pp
 See the